1. Field of the Invention
The present invention relates to a novel honeycomb core. More particularly, it relates to a honeycomb core which is lighter in weight and has higher strength properties than do conventional honeycomb cores.
2. Description of the Prior Art
Honeycomb cores has been conventionally formed, for example, of metal foil such as aluminum foil or various papers made of cellulose pulp, synthetic fiber, etc. A honeycomb core made of a suitable material selected from these materials is chosen depending on purpose of use. All of these materials are substantially isotropic.
On the other hand, when a honeycomb core is used as a lightweight material, a compressive stress and a shear stress both in the direction of the core axis are generally often produced in honeycomb ribbons constituting the honeycomb core.
Therefore, when the isotropic material described above is used in a honeycomb ribbon, the honeycomb ribbon is designed to have a strength which is expected to be required in the core axis direction. In this case, in the other directions, the honeycomb ribbon has performance characteristics equal to those in the axis direction though not a very large stress is exerted in the other directions. Therefore, the honeycomb ribbon has an excessive quality with respect to such directions.
In view of such production of stresses, the conventional isotropic materials such as metal foil and paper are not always suitable materials.
On such a line of thinking, as described in the 33th International SAMPE Symposium and Exhibition Vol. 33, pp. 73-77 (1988), an attempt has been made to produce a honeycomb core capable of making the most of performance characteristics of honeycomb ribbons, by using woven fabric which is an anisotropic material. The strength properties of woven fabric in the warp direction and those in the weft direction are usually different. When the structure of woven fabric is suitably chosen, the strength properties in the warp direction and those in the weft direction can be made the same, but even in this case, the strength properties in a direction making an angle of 45.degree. with the warp direction are different from those in the warp direction and those in the weft direction.
The utilization of woven fabric in a honeycomb ribbon, however, involves the following problems.
a) The directions of anisotropy are only the warp direction and the weft direction, i.e., two directions perpendicular to each other, and no optional anisotropy can be realized.
b) A honeycomb ribbon is generally a very thin material. When conventional woven fabric is used, only a relatively thick product can be obtained by comprising a ribbon of a piece of woven fabric and impregnating a resin into the ribbon. Moreover, since the amount of the resin impregnated is large, there cannot be obtained a honeycomb ribbon which is light considering its high strength properties. On the other hand, when the weight of woven fabric is reduced for lightening a honeycomb ribbon, the resulting woven fabric has a rough texture, so that no woven fabric having satisfactory physical properties can be obtained. Therefore, for attaining physical properties required of a honeycomb ribbon by the use of woven fabric, very thin yarn should be used, so that such an attempt is not practicable both technically and economically.
c) In woven fabric, yarns are bent at points at which a warp and a weft cross each other. Therefore, when a compressive stress is exerted in the direction of the axis of yarn, the yarns are further bent at the bent portions, namely, the woven fabric is deteriorated. Accordingly, it is impossible to make the most of the compressive strength of the yarn itself.
d) When in a honeycomb ribbon made of woven fabric, fibers are desired to be oriented at an angle of .+-.45.degree. with the longitudinal direction of the ribbon, a long piece of bias cloth should be used for obtaining such a long ribbon. But, the production of such a long piece of bias cloth is technically difficult. Even when it is possible, it is not economical.
The present invention is intended to provide a honeycomb core which does not have such defects and in which fibers have been oriented in an optional direction if necessary.